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Full text of "Text Book Of Mechanical Engineering"

Appendix IV.

955

Suppose then a machine has a velocity ratio of 4:1,   and  an
efficiency of 30 %r its real mechanical advantage would be

4x30        ____

I X IOO

Referring to example 42, p. 484, let us assume an efficiency of
60%.    Then

W         1120

rr = -—
r^        OO

•    V R =

II20X I0°

15

__
60 x 60         driver       5

d    foU°wer __ 1 1 20 >< 100x5       io'37
driver         60x60x15    ""     i

So, in actual practice, the pitch line diameters may he 3" and 31*2"
for pinion and wheel respectively.

Example 43 needs no  correction, because this is a question of
positive speed merely.

In example 44 let us again assume an efficiency of 6o°/0.    Now
TMA=i6:i=:VR

. *.   P M A = i6x-6 = 9-6: i

and Wx = 9*6 x 120 = 1 150 Ibs.

if the Weston block has an efficiency of 35 % (see P- 577) we nave

P M A = 20 x 35 = 7 : i
. • . Wj « 7 x 50 = 350 Ibs.

In like manner the real pull on the handle in the screw jack, p.
520, with an efficiency of 60° J0 for the worm and 35 °/0 for the screw,
or a total of '6 x '35 == -21 would be

P! = 20 -=- '21 = 95 Ibs.
which means that 10 tons could not be raised conveniently.

Pp. 574 and 873, Efficiency Curves. — Whenever experi-
ments are made upon machines in order to discover their frictional
loss and efficiency under all loads, the full set of curves shewn in
Fig. 895 ought to be plotted, to a load base The equations to

7 R